Polymer Chirality Modulates Phase Transitions at Liquid–Liquid Interfaces

In the poly(lactic acid) family, poly(l-lactic acid) (PLLA), poly(d-lactic acid) (PDLA), and mesomeric poly(d,l-lactic acid) (mPLA) have the same chemical constituents but distinct configurations. It is well-known that PLLA and PDLA could crystallize to homocrystals individually (HCL and HCD) or form stereocomplex crystals (SC) together. But it remains a debated question whether poly(lactic acid) mixtures with different chiralities can phase separate in melt or in solutions. Here, we study the phase transitions of PLLA/mPLA and PLLA/PDLA in dichloromethane (DCM) solutions at double emulsion interfaces. We find that crystallization is dominant in the low-molecular-weight (MW) mixtures, while liquid–liquid phase separation indeed occurs ahead of crystallization in the high-MW mixtures. The PLLA/mPLA double emulsion droplets transform into a spindle-like shape when the homocrystallization of PLLA is dominant in the low-MW region, while eyeball-like microcapsules form when phase separation is dominant in the high-MW region. The PLLA/PDLA double emulsion droplets are solidified to spherical microcapsules with dense stereocomplex crystallites at low MWs, while the multiphase patchy microcapsules form by sequential liquid–liquid phase separation, stereocomplex crystallization, and homocrystallizations at high MWs. We quantify a very weak repulsive energy of 0.0050kT (kT is the thermal energy at room temperature) between the l- and d-segments and depict the complete phase diagrams to provide insight into the multiple phase transitions. This study quantifies the interaction of poly(lactic acid) mixtures and provides a design principle for taking advantage of chiral polymers to modulate the microcapsule structures and properties via phase transitions at the liquid–liquid interface.